Depth of field refers to the area in front of and beyond the point focused upon in which things appear acceptably sharp in a photograph. Depth of field is really an illusion: Again, the point focused upon is sharp; everything in the scene closer to or farther from the camera is less sharp. Depth of field is based on the fact that our eyes can't differentiate between dead-on sharp and almost-sharp until the lack of sharpness reaches a certain degree. Within depth-of-field limits, objects in the photo will appear sufficiently sharp at normal viewing distances for the print size.
Depth of field affects every photograph. So it's a good idea to put it to work for you, rather than just leave it to chance. You have three controls over depth of field: aperture, focal length, and focusing distance. Smaller apertures increase depth of field, while larger apertures decrease it. Shorter focal lengths increase depth of field, while longer focal lengths decrease it. And greater focusing distances increase depth of field, while shorter focusing distances decrease it.
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Great depth of field is handy when you have several important subjects in a scene, each at a different distance from the camera. It's also useful when you want to get an entire three-dimensional subject sharp, rather than just a portion of it.